We have recently demonstrated that activated phagocytes employ the heme protein myeloperoxidase, H2O2 and Cl- to oxidize the aromatic amino acid L-tyrosine to the reactive aldehyde p-hydroxyphenylacetaldehyde. The product is formed in high yield, suggesting that other amino acids might similarly be oxidized to their corresponding aldehydes by phagocytes at sites of inflammation. We now present evidence for the generality of this reaction by demonstrating that neutrophils employ the myeloperoxidase-H2O2-Cl-system to oxidize nearly all of the common (-amino acids to generate a family of reactive aldehydes. Chemical characterization suggested that reactive carbonyl moieties were generated during amino acid oxidation by myeloperoxidase. The structures of amino acid-derived aldehydes were confirmed using a variety of mass spectrometric methods. Aldehyde production required myeloperoxidase, H2O2, Cl- and an amino acid; it was inhibited by heme poisons and catalase. Hypochlor ous acid (HOCl) was the apparent oxidizing intermediate because its addition to (-amino acids resulted in the formation of the anticipated aldehyde. Stimulated human neutrophils likewise generated aldehydes from all classes of (-amino acids by a pathway inhibited by heme poisons and catalase, implicating myeloperoxidase and H2O2 in the cell-mediated reaction. Aldehyde production accounted for up to 70 % of the H2O2 generated by stimulated neutrophils at physiological concentrations of amino acids. Collectively, these results suggest that amino acid-derived aldehydes represent a major class of reactive species generated by activated phagocytes. Aldehyde production during amino acid oxidation may constitute an important mechanism for signaling and the selective damage of critical targets at sites of inflammation and vascular disease.